1. Field of the Invention
The present invention relates to a process for preparing microcapsules containing a target material, the microcapsules produced thereby, and compositions containing these microcapsules. More particularly, the present invention relates to a polymerization process for microencapsulation.
2. Brief Description of the Prior Art
A variety of methods for the preparation of microcapsules are known. In general, as an initial step one fluid is dispersed within another, the two fluids being immiscible, or nearly so, and in any case forming separate phases. A common example is the oil-in-water (o/w) dispersion, although water-in-oil (w/o) dispersions are also well known, and the only physical criterion for selecting the fluid pair is mutual incompatibility at the selected temperature and pressure. Often a surface-active dispersant or protective colloid, such as polyvinyl alcohol in the case of oil-in-water dispersions, is dispersed or dissolved in the continuous phase to stabilize the dispersion. The ultimate object is to form a capsular wall or shell around the dispersed phase droplets or particles, the dispersed phase being formed by or containing a target material which is to be encapsulated. Microencapsulation techniques are reviewed in I. E. Vandergaer, Microencapsulation (Plenum Press, London 1974). The methods for forming the wall can be broadly divided into physical and chemical techniques.
The physical technique, complex coacervation, involves precipitation of a polymeric species at the interface between the continuous and discontinuous phase. For example, gelatin, dissolved or dispersed in a continuous aqueous phase at controlled temperature and pH, can be coacervated or precipitated at the interphase between the aqueous phase and a dispersed organic fluid phase, by reaction with an anionically charged colloid, such as gum arabic, vinyl acetate-maleic anhydride copolymer, sodium alginate, polyacrylic acid, or the like. The walls or shells formed at the interface can be subsequently hardened by physical or chemical treatment, such as disclosed in U.S. Pat. No. 2,800,457. Coacervation processes typically require careful control over process conditions such as reactant concentrations, temperature, and pH, and employ substantial proportions of a relatively expensive material, gelatin, in forming the capsule shells. The processes are complex, and give microcapsules which typically have poor water resistance.
Numerous chemical techniques for forming the microcapsule shells have also been proposed. For example, urea and formaldehyde or a urea-formaldehyde precondensate can be dispersed in a continuous aqueous phase, and subsequently induced to react to give a urea-formaldehyde condensate which forms encapsulating shells around a dispersed phase containing the target material. Urea-formaldehyde microencapsulation is taught, for example, in U.S. Pat. Nos. 3,016,309 and 3,796,669. The use of polymeric species such as gum arabic, polyacrylic acid, alkyl acrylate-acrylic acid copolymers, and hydrolyzed poly(ethylene-co-maleic anhydride) to modify the properties of urea-formaldehyde shells is reviewed in U.S. Pat. No. 4,552,811. Processes in which the microcapsule shell polymer is polymerized in either the continuous phase or the discontinuous phase are often referred to as "in-situ" techniques.
The in-situ shell-forming materials can be included in the discontinuous phase. For example, U.S. Pat. No. 4,626,471 discloses in-situ polymerization of certain multifunctional epoxy resins using polyamine curing agents. The epoxy resin and amine are emulsified in aqueous solution, and the temperature is elevated to promote cure of the resin. The cured resin migrates to the interface to form the shells of the microcapsules.
Another set of methods for encapsulating target materials involves interfacial polymerization, the polymeric shell being polymerized at or near the interface between the continuous and discontinuous phases. Typically, the polymerization reaction is a condensation or addition reaction involving two types of difunctional monomer, the first being dissolved or dispersed in the continuous phase, the second being dissolved or dispersed in the discontinuous phase.
For example, U.S. Pat. No. 4,622,267 discloses an improved interfacial polymerization technique for preparing microcapsules for carbonless copy paper. The target material (color-former) is initially dissolved in a good solvent and an aliphatic diisocyanate soluble in the good solvent/color former mixture is added. Subsequently, a poor solvent for the aliphatic diisocyanate is added until the turbidity point is just barely reached. This organic phase is then emulsified in an aqueous solution, and a reactive amine is added to the aqueous phase. The amine diffuses to the interface, where it reacts with the diisocyanate to form polymeric polyurethane shells. A similar technique, used to encapsulate salts which are sparingly soluble in water in polyurethane shells, is disclosed in U.S. Pat. No. 4,547,429.
An interfacial photopolymerization method is disclosed in U.S. Pat. No. 4,532,183. In this addition polymerization technique, free radical polymerizable monomers are present in both a continuous aqueous phase and a discontinuous oil phase. The aqueous phase can include a hydroxyalkyl acrylate or methacrylate while the oil phase can contain copolymerizable ethylenically unsaturated oil soluble monomer such as an alkyl acrylate. Photoinitiator can be added to either phase, and a polyfunctional isocyanate prepolymer is preferably added to the oil phase to enhance shell formation.
Microcapsules have been used to encapsulate a great variety of target materials. The most important commercial use of microencapsulated materials has been in the manufacture of carbonless copy paper. Typically, a colorless dye precursor or color-former such as crystal violet lactone is encapsulated in microcapsules having fairly rigid shells, and a slurry containing the microcapsules is coated onto the back of a first sheet (CB sheet). The face of a second sheet is coated with an acid, color developing material such as an acidic clay or a phenolic resin (CF sheet). The sheets are manufactured into a form with the CB sheet over the CF sheet. Pressure on the CB sheet, such as that generated by the ball of a ballpoint pen, ruptures the shells of the microcapsules to free the dye-precursor to react with the color developer and form a copy of the original on the CF sheet.
Other microencapsulated target materials have included agricultural chemicals, food for newly hatched fish, pharmaceuticals, pesticides, flavorings, scents, adhesives, toners for xerography, fertilizers, inks, toxic salts and crosslinking agents and other reactive chemicals. The nature of the application strongly influences the characteristics of the polymeric shells. For example, in the case of encapsulated pharmaceuticals, sustained, gradual release of the target material from the microcapsules may be desired, and the shell porosity and/or biodegradability could be controlled to achieve the desired release kinetics. In the case of carbonless copy paper, the microcapsules must be rigid for easy rupturability, and relatively impervious to diffusion by the color-former for stability.
Microencapsulation techniques are often directed to the problems associated with encapsulating specific target materials and cannot be easily generalized to different types of target materials. For example, the in situ polymerization of urea-formaldehyde precondensates to encapsulate dispersed oil phase droplets containing color former cannot be easily adapted to target materials requiring water-in-oil encapsulation, such as water-soluble vitamins. There is a need for an encapsulation process of sufficient breadth such that a wide variety of target materials can be successfully encapsulated. In particular, there is a need for an encapsulation process which can be used to encapsulate both hydrophilic target materials, such as water-soluble target materials, and hydrophobic target materials, such as oil-soluble target materials.